Flush Kanban logs a bit more efficiently

Problem/Motivation

Currently the code for flushing the logs isn't very efficient, since it loads all the entities into memory.

It should be more memory efficient, ideally using something like the Batch API to process a set of logs at a time.

Steps to reproduce

Attempt to flush a kanban log of thousands will likely result in the page crashing.

Proposed resolution

Use a more efficient API like the Batch API to flush the entities.

Remaining tasks

Use the Batch API to flush the data.